The presence of air bubbles or foam in hydraulic fluids has virtually always been considered a negative condition because of the so-called “sponginess” shown by such liquids when applying hydraulic pressure to activate mechanical devices. See, e.g., Hodges, P., Hydraulic Fluids, John Wiley and Sons, 1996; Williams, L. “Aeration,” GF-5 Video Archives (www.gf-5.com), 2008, Lubrizol, which relates to the same in automotive engines.
In engine oil, formation of foam in the operating engine by aeration is undesirable at hydrodynamically lubricated surfaces because it markedly increases potential for wear. See, e.g., Choi, J-K, et al., “Effect of Oil Aeration on the Minimum Oil Film Thickness and Reliability of Engine Bearings,” SAE, Warrendale, Pa., Paper No. 932785, 1993; Nikolajsen, J. L., “Viscosity and Density Models for Aerated Oil in Fluid-Film Bearings,” Tribology Transactions, Vol. 42(1), pp. 189-191, 1999; Chun, S. M., “A Parametric Study on Bubbly Lubrication of High-Speed Journal Bearings,” Tribology International, Vol. 35, pp. 1-13, 2002; Jang, S., et al., “Study on the Effect of Aerated Lubricant on the Journal Traces in the Engine Bearing Clearance,” International Journal of Automotive Technology, Vol. 6(4), p. 421, 2005.
Foam has been even more of a problem when recent engine designs have required engine oil to also function as a hydraulic fluid, particularly foam produced from release of any entrained air in the oil. See, Hodges, P., supra; Porot, P., et al., “A Numerical and Experimental Study of the Effect of Aeration of Oil in Valve Trains Equipped with Hydraulic Lash Adjuster,” SAE, Warrendale, Pa., Paper No. 930997, 1993. These newer hydraulic functions such as cylinder deactivation and variable valve timing are more demanding of the engine oil than prior use of the oil to serve in such applications as in hydraulic valve lifters. Note, Hodges, P., supra.
As a consequence, a strong need has developed to distinguish among engine oils regarding resistance to the following:                1. Foam formation and foam retention in the crankcase;        2. Absorption of entrained air during engine operation; and        3. Liberation of entrained/dissolved gas when exposed to any relatively sudden decrease of pressure.        
There are a number of engine operating conditions that can produce pressure decreases. For example, pressure drop is encountered by oil in the process of emerging from an oil gallery through which it has been pumped under the pressure required to overcome viscous resistance.
Thus far, according to a personal communication from 2009 with King, T., ILSAC/Oil Chairman, regarding past efforts to find an acceptable aeration test by the industries involved in the improvement of engine oil, this growing need to control foaming from entrained air has been said to be essentially unmet, and this is despite strong efforts on the part of the petroleum and additive industries. A testing exception is an older engine test, ASTM D 6894-08, “Evaluation of Aeration Resistance of Engine Oils in Direct Injected Turbocharged Automotive Diesel Engines,” which, however, is expensive, imprecise, time consuming, and of limited availability.
Other art is known. See, Selby et al., U.S. Pat. No. 5,824,886 (Oct. 20, 1998), which discloses a foam tester; Hildebrandt et al., U.S. Pat. No. 6,009,748 (Jan. 4, 2000), which discloses a rapidly cyclable foam testing oven; ASTM D 892-06, “Standard Test Method for Foaming Characteristics of Lubricating Oils,” and ASTM D 6082-06, “Standard Test Method for High Temperature Foaming Characteristics of Lubricating Oils.” Compare, Selby, T., et al., “A New Approach to the Determination of Extracted Profoamants from Elastomeric Sealants,” presented at the 12th Esslingen Colloquium, Jan. 11-13, 2000, Esslingen, Germany.
Among drawbacks in such art is that a large, 26-pound sample must be employed to effectively run and obtain results from the engine aeration test. Sample size in the known bench foaming tests is required to be several hundred milliliters (mL) of sample. Also, the relationship among foaming, aeration, and gas entrainment is relatively unexplored but important to the use of lubricants for hydraulic fluid and/or engine oil applications.
It would be desirable, accordingly, to provide a simple bench test that can illuminate the foaming tendencies of air-entraining oil under a pressure drop, notably, for example, before the oil enters the field. It would be desirable to provide for greater accuracy and precision with such a test as well as to provide a bench test that is more revealing and perhaps predictive of future performance of an oleaginous liquid during and after use than that which is provided by the known art. It would be desirable to provide a bench test that would employ a smaller sample size than tests of the known art. It would be desirable to provide an alternative to the art.